Castable ceramic materials made of fused silica-aggregate and calcium aluminate cement are widely used in many high temperature (1000.degree. F.-1700.degree. F.) environments because of their low cost and their properties of compression strength, hardness, heat resistance, dimensional stability and rapid castability to net shape. However, their use has been limited because of their low modulus of rupture and brittleness, which historically has lead to failures due to cracking. Other stronger castable ceramics, such as mullite based materials are being developed, but these materials are also relatively weak and brittle and have their own drawbacks, such as excessive shrinkage during drying and curing. All of these materials would be more useful if a way were found to increase their load bearing capacity.
One valuable application for ceramic materials has been in forming dies for superplastic forming, primarily of titanium and aluminum alloys, but also some corrosion resistant steel alloys, nickel alloys and superalloys. The desirable properties of low cost, rapid production and excellent high temperature stability make possible the fast, accurate and low cost production of dies using castable ceramic materials. U.S. Pat. No. 5,467,626 issued on Jun. 4, 1996 to D. G. Sanders teaches an innovative freestanding ceramic die made with these materials. The disclosure of this patent is incorporated herein by reference.
However, the durability of ceramic dies is poor. It is uncommon for a die for a part of average draw depth to produce 20 parts before it fails and must be replaced, so ceramic dies are used primarily for rapid prototyping and limited production quantities of parts. Durability can be increased by increasing the thickness of the die, but excessive thickness increases the mass of the die and increases the handling difficulty. More importantly, thick walled dies are difficult to manufacture because they tend to crack during the initial drying stage, which is a process in which heat is released in an exothermic reaction, causing extreme thermal stress on the die. Also, during curing and firing, shrinkage and trapped moisture can cause cracking of a die that has survived the drying process. During part production, the heat-up time for raising the temperature of a massive superplastic forming die to 1650.degree. F., a typical superplastic forming temperature for titanium, is significantly longer than it is for a standard "thin" walled die. Accordingly, there has been a continuous effort by many workers in the art to improve the modulus of rupture of ceramic superplastic forming dies.
The attempts that have been made to solve the tensile weakness and brittleness problem with ceramics have been primarily of two types: 1) providing external compressive support to the structure and 2) mixing fibers of a reinforcing material into the castable materials and allowing the ceramic to cure with the fibers in place.
External compressive support is a common stratagem and in fact was first used in the form of a steel containment box as a safety precaution because of the fear of explosive bursting of the ceramic die under high forming gas pressure and high compressive loading by the press on the lid to seal the pressurized forming gas in the die cavity. Steel containment boxes continue to be used despite the proven safety of freestanding ceramic dies. One illustration of such a containment vessel is seen in U.S. Pat. No. 4,584,860 issue to Kirke Leonard.
Although steel or CRES enclosed ceramic dies work well and have adequate strength, they are substantially more costly to produce and the removal of the ceramic material from the containment box after the useful life of the die has ended is difficult and laborious. Parts made using the steel box enclosed ceramic dies are historically more expensive since a larger blank from which the part is made is needed to attain a seal around the forming chamber periphery, resulting in large amounts of scrap/waste that must be trimmed off of the formed part. Other more innovative forms of compressive external support of the die have been tried successfully. One such technique is shown in U.S. Pat. No. 5,683,608 to Marc Matsen et al. entitled "Ceramic Die for Induction Heating Work Cells". This technique uses fiberglass rods cast into the ceramic die and used as tie rods to hold external phenolic blocks in compression against the four sides of the die. This scheme works well but its use is restricted by the temperature limitations of the reinforcing materials.
The use of reinforcing fibers mixed into the castable ceramic material when it is mixed up for casting is a concept that has been explored by numerous ceramic investigators. The material of which the reinforcing fibers is made has high tensile strength and the theory behind this approach is that the tensile load would transfer to and be borne by the fibers, thereby improving the modulus of rupture and tensile strength. Experience with reinforcing fibers in ceramics is encouraging: although there has not been a significant increase in the modulus of rupture, there has been some improvement in toughness. Some questions remain whether the fibers bond sufficiently with the cement, or whether the amount of strain required in the ceramic material before tensile stress can be transferred to and borne by the fibers exceeds the strain capability of the brittle ceramic material. The large difference in coefficient of thermal expansion between the CRES fiber and the ceramic material can produce internal stresses within the die that can be deleterious to its survival in a hot, high stress application.